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1. IL-0344 Linode's US-West data centers in Fremont and Santa Clara lost connectivity as the fibre circuits between them failed

   Linode2023

   Symptoms

   On 12 February 2023, customers in Linode's US-West region could not reach their Linodes. Santa Clara customers lost Internet connectivity after a change made at 8:45 UTC until it was reverted at 10:13 UTC, and later in the day the connection between the Fremont and Santa Clara data centers was cut completely, leaving both with degraded service or a full outage until 16:25 UTC; customers could also have trouble provisioning, powering down or rebooting services.

   Network unreachable

   Trigger

   A single link of the A-side fibre circuit between Fremont and Santa Clara went down on 11 February, the whole B-side circuit followed on 12 February, and during troubleshooting with the hardware vendor a non-intrusive command triggered a rare bug that took down all A-side links.

   Root cause

   Software defect

   Component

   Backbone network (WAN)NIC, cable and switch port link

   Contributing factor

   Overlapping failuresHarmful manual action

   Detection

   The response team began investigating when the A-side link went down on 11 February. The status page opened at 10:22 UTC on 12 February to investigate reports of connectivity issues in the US-West region.

   Remediation

   • Reverted the change that removed internet traffic from the Santa Clara border routers
   • Performed an emergency code upgrade of both circuits, restoring connectivity between the sites
   • Restored the B-side path with the hardware vendor and fiber provider, regaining full redundancy

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2. IL-0256 Basecamp was fully down twice in two days as hardware load balancers crashed and routers lost sync after failover

   37signals2020

   Symptoms

   On Tuesday 1 September 2020 Basecamp could not be loaded at all for 17 minutes, from 9:51am to 10:08am Central Time (14:51–15:08 UTC), and nothing seemed to work. On Wednesday 2 September it happened again for 15 minutes, from 10:50am to 11:05am Central Time (15:50–16:05 UTC).

   Service outageLink flappingHigh latency

   Trigger

   On 1 September the network links between the data centers went offline, and during the recovery a hardware load balancer crashed. On 2 September the primary load balancer in Virginia crashed again, and later its secondary crashed too.

   Root cause

   Software defect

   Component

   On-premises load balancerBackbone network (WAN)

   Contributing factor

   Overlapping failuresNo headroom

   Remediation

   • On 1 September, intervened by hand to get the network routers working with the standby load balancer
   • On 2 September, fixed the same router and load balancer desynchronization by hand

   Action items

   • Find the cause of the load balancer crash with the vendor and apply its bugfix
   • Replace the hardware load balancers
   • Reroute the redundant cross-datacenter network paths for proper circuit diversity
   • Evaluate moving from hardware to software load balancers, which can be deployed in minutes
   • Evaluate decentralizing the load balancer architecture to limit the impact of one failure
   • Assess the systems for resilience, fragility and risk, and review the assessment process itself

   Lessons

   Basecamp plans for failures like these and practises responses to catastrophic failures in its live systems, but three obscure issues in five days chained into overlapping failures. What caught the team by surprise were cascading failures that exposed unexpected fragility and hard paths to recovery, which Basecamp says it can prepare for.

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